Report on DELP 1988 Cruises in the Okinawa Trough : Part 3. Crustal structure of the southern Okinawa Trough
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概要
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A refraction seismic experiment was conducted in the southern Okinawa Trough to study the crustal structure of a back-arc basin. We deployed 18 ocean bottom seismometers (OBSs) every 11km along a 195-km profile. We shot three large explosives (600kg, 500kg, and 450 kg) and 50 small explosives (50 kg and 25 kg). The explosions were complemented by shots from an array of a 9-liter airgun and a 16-liter airgun. The average shot interval of the explosives was 3.8 km, and that of the airguns was 150m. The analysis of the refraction data, using the τ-p method and two-dimensional ray tracing forward modeling of the seismic wave field, showed no evidence of a young oceanic crust even beneath the central axis of the southern Okinawa Trough. A 2-3-km thick sediment was underlain by a layer with a P-wave velocity around 6 km/s. The Moho depth was more than 18 km below the sea surface according to the phase which was propagated in the lower crustal structure, although no clear first arrival of the refraction phase of the uppermost mantle was observed. The distribution of the P-wave velocity and the thickness of the crust indicated that the crust of the southern Okinawa Trough is more similar to that of a continent than to that of an ocean. Our data support the idea that the southern Okinawa Trough is in the stage of incipient rifting of the back-arc basin accompanied by a slight thinning of the continental crust.1988年7月に南部沖縄トラフで, 18台の海底地震計を用いた屈折法地震探査が行われた.南北195kmの測線上で3発の大発破(600, 500, 450kg)と50発の小発破(50, 25kg)を行った.更に,上部地殻を精密に調べるために, 91と161のエアガンアレイを50～70秒間隔で発振して,高密度の屈折地震波記録を得た.海底地震計の間隔は平均11km,平均火薬発破間隔は3.8km,平均エアガン発振間隔は150mであった.
- 東京大学地震研究所,Earthquake Research Institute, University of Tokyo,Department of Earth Sciences, Chiba University,Earthquake Research Institute, The University of Tokyo,Department of Marine Mineral Resources, Tokai University,Geophysical Institute, The Universiの論文
- 1991-06-28
著者
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平田 直
The Earthquake Research Institute The University Of Tokyo Tokyo 113-0032 Japan
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Kaneda Yoshiyuki
Japan Agency For Marine‐earth Sci. And Technol. Yokosuka Jpn
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Baba H
Tokai Univ.
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Katao H
Kyoto Univ. Kyoto Jpn
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Hirata Naoshi
Univ. Tokyo Tokyo Jpn
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Hirata Naoshi
Department Of Earth Sciences Chiba University
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Hirata N
Univ. Tokyo Tokyo Jpn
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